20 



GEORGE E. NICHOLS Vol. IV, No. I 



i. Climatic Habitat Factors 



These include essentially all factors which are associated with atmospheric 

 conditions— conditions chiefly of moisture, temperature, and light. 18 With 

 reference to the extent of their influence, climatic factors may be either 

 regional or local ; with reference to the continuity of their effect, they may be 

 either stable or unstable ; changes in climate, generally speaking, may be either 

 rhythmical or progressive. 



Regional Climatic Factors. These are atmospheric conditions of wide- 

 spread general uniformity. They are the factors associated with climate as 

 ordinarily conceived — the ones with reference to which the climatic regions of 

 the earth are delimited. 



Local Climatic Factors. Here are included modifications of the regional 

 climate due primarily to variations in topography or to the position in relation 

 to one another of land and water bodies. Familiar examples are seen in the 

 different atmospheric conditions of north- as compared with south-facing 

 slopes, of exposed headlands as compared with near-by protected situations, 

 of ravines as compared with level or rounded uplands. 19 Their influence is 

 most pronounced in regions where the topography is varied and where, at the 

 same time, the climate as a whole is relatively unfavorable to vegetation ; but, 

 strictly speaking, no two spots on the face of the earth have exactly the 

 same climate. 



Rhythmical Changes in Climate. Absolute climatic stability, of course, 

 does not exist. Every climate is characterized by those regularly recurrent 

 alterations which mark the change from day to night and from one season to 

 another. A climate in which these rhythmical cycles of daily and seasonal 

 change continue essentially unchanged from year to year may be regarded as 

 relatively stable. 



Progressive Changes in Climate. Progressive changes in climate result 

 from the gradual and progressive alteration in the character of any particular 

 climatic factor or group of factors over a period of years — changes such as are 

 caused by increasing aridity or cold. These changes may be brought about 

 within a relatively brief period of time, for example, within a century; 20 or 

 they may extend over long periods of time, typically geological periods, as, 

 for example, the changes (considered in the large) which have taken place 

 since the last glacial era. Relatively rapid progressive changes may exercise 



18 It might perhaps be more correct to speak of these as meteorological habitat fac- 

 tors, for two reasons, (i) Strictly speaking, climate is an effect of which certain 

 meterorological factors are the cause. (2) Light is a meteorological factor which is not 

 always regarded as a climatic factor. Or they might be termed atmospheric habitat 

 factors. For various reasons, however, the term climatic habitat factor seems preferable. 



19 In a sense, the influence of shade (see under biotic factors) represents a local 

 modification of the regional climate. 



20 See in this connection the article by C. E. Brooks : Secular variation in climate, 

 Geog. Rev., 11: 120-135, f . 1 -f 3 maps 1921. 



